In the early stage when the video cassette recorders were developed, chromium-plated copper, austenitic stainless steel, ceramic-coated or alumite-treated cast aluminium had been used as a material for head drum and/or other parts of VCR on which the video tape travels.
However, as those materials were found to have many disadvantages in properties, manufacturing technology and cost, they are no longer used and mostly aluminium alloys are being used at present.
However, stabilized tape travelling is hard to be expected by using conventional aluminium alloy developed for die casting, because the aforesaid parts made from the alloy have many defects on their surfaces due to pinholes, rough and volumnious eutectic compounds and precipitates formed during the casting and heat treatment process.
Therefore, in order to solve such problems, many studies have been conducted and various aluminium alloys, e.g. Al-Si and Al-Cu alloys, have been developed.
Aluminium alloys of Al-Si group developed by adding the alloying elements such as Cu and/or Mg are usually for use in casting, but some of them are for forging.
Although these alloys are widely used for their excellent abrasion resistance and their free cutting characteristics without changing mechanical properties, they still have such defects that precision of finishing is inferior because the tools used in finishing work suffer high abrasion loss, deteriorating the surface smoothness, and magnetic tapes might be hurt by silicon particles with super hardness exfoliated from the alloys.
On the other hand, Al-Cu aluminium alloys with Mg and Ni added thereto and for use in forging also have several disadvantages such as poor abrasion resistance with insufficient hardness, inferior free cutting ability owing to easy curling of finishing chips and higher cracking rate in cold forging, while they can assure more stabilized travelling of video tape and better durability of machine tools.
Generally, materials for head drum of VCR should be non-magnetic ones, guarantee the video tape's travelling to be stabilized without damaging it and have high reliability regardless of long use or change of environments.
In order to satisfy the above conditions, materials for head drum of VCR is required to have following properties;
(1) Proper hardness value PA1 (2) Excellent abrasion resistance PA1 (3) Uniformity of internal structure PA1 (4) Smaller thermal expansion coefficient PA1 (5) Excellent workability guaranteeing proper surface roughness PA1 (6) Excellent cold-forgeability or cold-castability PA1 (7) Low manufacturing cost